1. Technical Field
This invention relates generally to a method and apparatus for reducing the effects of ground float in an electronic control circuit, and more particularly to a method and apparatus for ensuring accurate readings in a vehicle electronic controller.
2. Discussion of the Related Art
Precise management and control of operating parameters have allowed electronic controller to greater improve the operating performance of the systems they control. Consequently, electronic controllers are used extensively in modern-day vehicles. Some are specialized controllers that control a single system and some are general controllers that control a variety of general functions. For example, there are engine controllers that monitor fuel, air and spark behavior to optimize fuel economy and maximize engine performance, and body controllers that monitor vehicle functions, such as wheel and vehicle speed, and which control a variety of vehicle functions such as active suspension and illuminating warning lights.
While controllers have allowed greater control of vehicle performance, they can only achieve optimal control by precisely monitoring and controlling parameters. However, one persistent and difficult problem associated with precisely measuring parameters for use by a vehicle electronic controller is the fact that any measurement, by its nature, is an imprecise, relative measurement. That is, the act of measuring involves gauging a parameter against a reference or criteria. In vehicle controllers, the reference point usually used in measurements is ground. Ground is commonly assumed to equal a "zero" level, so that the measurement of signals involves determining their relative magnitude over ground. Unfortunately, in vehicles, ground does not always remain at zero. Depending upon the characteristics of the vehicle electrical system, ground can varying dynamically by several volts during the operation of the controller. This phenomenon is commonly referred to as "ground float", because the absolute level of the ground varies, or floats. Therefore, a signal measured as having a magnitude of "X" is really a signal having a true magnitude of "X over ground". If ground varies, it is obvious that the true magnitude of the signal may be clouded by the fact that it is measured relative to a floating ground.
Several ways to reduce the effects of ground float have proved helpful but not wholly effective. One way of reducing the effect of ground float on an electronic signal measurement is to make the measurement less sensitive. By measuring using a coarser measurement criteria, the variations in ground do not affect the final measurement as much. However, using a coarser scale also reduces the ability of the controller to finely control based upon measured parameters. Another way of reducing ground float is to electrically isolate ground. This often involves complex isolation circuits which attempt to protect ground from being influenced by electrical system variations. Unfortunately, isolation circuits can be expensive and still do not completely protect ground from being affected by circuit variations.